All images not accounted for...

;Blitz normally passes variables by value, with no way to pass by reference
;(short of using Globals, which negates the point of explicitly passsing them)

a=100
b=10
Print "pre: "+a   ; displays 100
Print "pre: "+b   ; displays 10
foo(a,b)
Print "post: "+ a ; displays 100
Print "post: "+b  ; displays 10
Print"---"

;However, Types are always passed by reference

Type bartype
 Field a
 Field b
End Type

bar.bartype=New bartype

bar\a=100
bar\b=10
Print "pre: "+bar\a   ; displays 100
Print "pre: "+bar\b   ; displays 10
foobar(bar)
Print "post: "+ bar\a ; displays 200
Print "post: "+bar\b  ; displays 11

WaitKey
End


Function foo(a,b)
 a=200 ; these changes will not matter outside this function
 b=b+1
End Function

Function foobar(baz.bartype)
 baz\a=200 ; these changes *will* matter outside this function
 baz\b=baz\b+1
End Function

This shows how the variables passed to foo() are not affected outside of foo() since only the *values* of a & b are passed to it, not "the real" a & b.

I would have expected your function to cause a problem when it tried to FreeImage a handle that had already been freed, but that doesn't happen:

Graphics 800,600
FreeImage 12345 ; just a number picked out of thin air
FreeImage 12345 ; even worse, we're freeing this nonexistent handle twice!
hdl=CreateImage(32,32)
FreeImage hdl
FreeImage hdl ; Blitz has no problem with this

Print "No boom. Press a key."
WaitKey 
End

(BTW, there's a similar discussion about "missing memory" for 3D at http://blitzbasic.com/bbs/posts.php?topic=25942 )